(* examples from the text *)

fun repeatEach([]) = []
  | repeatEach(a::rest) = a::a::repeatEach(rest);

fun sum([]) = 0
  | sum(a::rest) = a + sum(rest);

fun switchPair([]) = []
  | switchPair((a,b)::rest) = (b, a)::switchPair(rest);





(* 2.2.4 -- in class*)
fun count([]) = 0
    | count(a::rest) = 1 + count(rest);

(* 2.2.5 -- in class *)
fun findIth([], i) = ~1
  | findIth(a::rest, 0) = a
  | findIth(a::rest, i) = findIth(rest, i-1);

(* 2.2.6 -- in class *)
fun reverse([]) = []
  | reverse(a::rest) = reverse(rest)@[a];


(* Ex 2.2.10 -- in class *)
fun listify([]) = []
  | listify(a::rest) = [a]::listify(rest);



(* Ex 2.2.12 -- in class *)
fun splitList([]) = []
 | splitList((a,b)::rest) = 
     let val (yy,zz) = splitList(rest);
     (a::yy, b::xx)
end;

(* Ex 2.2.13 -- homework  *)
fun tuplify([], []) = []
  | tuplify(a::rest, []) =  ?? 
  | tuplify([], b::rest) =  ??
  | tuplify(a::aRest, b::bRest) =  ??

(* Ex 2.2.14 -- in class *)
fun sumPairs([]) = []
  | sumPairs((a,b)::rest) = (a + b)::sumPairs(rest);


(* Ex 2.2.15  -- homework *)
fun sumPairList([]) = (0, 0)
  | sumPairList((a,b)::rest) =
    let val (c, d) = sumPairList(rest)
    in
        ??
    end;

(* For the next HW assignment (due Sept 12) *)


(* Ex 2.4.14 -- homework *)
(* The second parameter is a list of lists *)
fun addToEach(x, []) = ??
  | addToEach(x, aa::rest) = ??     (* aa is itself a list ! *)

(* Ex 2.4.15  -- homework *)
fun powerset([]) = ??    (* Think carefully: What is the powerset of the empty set? *)
  | powerset(a::rest) = ??